During the past few years, visible light-emitting diodes (LEDs) using vertical InGaN/GaN nanowires have for example been fabricated, containing a p-n junction and collectively connected in parallel.
Thanks to their potential intrinsic properties (good crystalline quality, relaxation of the stresses on the free vertical surfaces, good efficiency of extraction of the light, etc.), nanowires are considered as very interesting candidates for overcoming the difficulties currently encountered with conventional GaN LEDs fabricated in a planar structure (2D).
Two nanowire LED approaches, based on different growth techniques, have already been provided and are known to those skilled in the art.
The first approach consists in epitaxially growing GaN nanowires containing InGaN quantum wells in an axial configuration using Molecular Beam Epitaxy (MBE). The devices fabricated using these nanowires have produced interesting results in the green spectral range. The processed chips of 1 mm2 can emit around 10 μW at 550 nm for a continuous operating current of 100 mA.
FIG. 1 illustrates such a configuration showing nanowires NTi on the surface of a substrate 11, typically made of silicon, in contact with a lower contact of the n type 10, the upper contact of the p type being provided by a transparent layer 12 and the contact being made via a thick p contact pad 13. The nanowires NTi with an axial structure comprise an n-doped region that can typically be formed from n-doped GaN, an active region ZA made of InGaN or having a structure with quantum wells, and a p-doped region that can be formed from p-doped GaN.
With the molecular beam growth (MBE) technique, some non-uniformities appear owing to random nucleation mechanisms, but typically an optical power on a single wire emitting at 550 nm of 50 nW has been obtained, this being 5 W/mm2 with around a hundred nanowire emitters/mm2.
More recently, the MOCVD (metal-organic chemical vapor deposition) growth technique has enabled the formation of InGaN/GaN nanowires containing a radial LED structure (Core/Shell configuration).
FIG. 2 illustrates this type of configuration in which nanowires NTi are formed on the surface of a substrate 11 covered by a nucleation layer 21, a lower contact layer 10 also being provided. The localized epitaxy takes place through a mask 20. The structure of the nanowires is of the core/shell type. The core 30 can comprise an n-doped GaN material, typically with a doping level of 1019 cm−3, a quantum well structure with alternating layers, which can respectively be of non-doped GaN and InGaN, and finally a shell 31 which can be composed of a p-doped GaN layer typically with a doping level of 1019 cm−3.
A dielectric layer 40 provides the isolation between the lower and upper contacts.
The upper contact is provided via an upper layer 50, which is conductive and transparent at the emission wavelength of the photoconductive structure. A metal contact layer 60 is also included in order to provide a mirror function.
In this approach, since the structure LED uses a Core/Shell configuration, the surface area of the active region is greater than in a 2D nanowire LED approach comprising planar structures.
Nevertheless, the applicant started from the observation that epitaxial processes, such as the MOCVD process, because of the consumption of all the gaseous species, generate edge effects detrimental to obtaining uniform components, and discontinuities in the regions of growth of the nanowires, more precisely, a variation over the wafer of the growth surface area fraction.
Indeed, notably in the case of LEDs, the nanowires fabricated by epitaxy for example of GaN on growth patterns may be defined by the standard techniques of microelectronics.
These patterns are assembled on the substrate in the form of regular patterns in regions that are compact, square, round, hexagonal, triangular, etc. which define the active surface of the LED. These regions have dimensions defined by the needs of the final user of the product, for example 100 μm by 100 μm, 350 μm by 350 μm, 1 mm by 1 mm, 3.5 mm by 3.5 mm, 10 mm by 10 mm. Each growth region is separated from its neighbor by a spacing whose dimension is adjusted to at least allow the passage of the metal power supply connections for the LEDs and the dicing by sawing of the substrates, or another dicing technique.
These spacings between the growth regions, in which no epitaxy is desired, cause the appearance of multiple defects:
non-uniformities in heights and in morphologies, in the networks of wires, associated with the discontinuities in the growth regions;
unorganized growths of nano-crystals for example of GaN which are killer defects for the circuits at the end of the technological steps;
spurious depositions of InGaN which cover the surfaces of the wafers with conducting materials.
FIG. 3 shows a view taken of a set of nanowires and an array of defects appearing in the regions with no growth, which photograph was taken on a scanning electron microscope, which highlights spurious growths Crpa, depositions of residues of growth Rcr and/or walls of nanowires with non-uniform dimensions around the periphery.
A solution may advantageously be envisioned notably allowing the defects to be eliminated by rendering the entire surface of a substrate uniform in terms of growth of nanowires and by selecting an elementary region of nanowires from amongst the larger set of nanowires, thus providing a functional support having uniform nanowires and while at the same time carrying out the selective elimination of certain nanowires in order to notably clear areas dedicated to ohmic contacts. Nevertheless, the removal of sub-assemblies of nanowires may prove to be tricky to implement between sub-assemblies of nanowires to be conserved.